1. Field of the Invention
The present invention relates to an isostatic molding method for ceramic powders, metal powders and their composites, more precisely, to a semiconductor package for which is used high-melting-point metal powders of W, Mo and the like, composites such as Cu/W, Cu/Mo, W/Ni/Cu, W/Ni/Fe and the like, or composites such as Mo/TiC, Al/SiC and the like, and also to an isostatic molding method for producing heat-radiating substrates for semiconductor packages.
2. Description of the Related Art
Heat radiation from packages of semiconductor devices that are used in communication-related appliances such as portable telephones and others is the recent important theme in the art. Heat-radiating members on which those semiconductor devices for microwaves are to be mounted are being in demand. For the heat-radiating members, metallic materials of aluminum, copper and the like may be taken into consideration in view of their heat conductivity. However, as greatly expanding with heat, members of such metallic materials are often problematic in that, when bonded to semiconductor chips of silicon and the like or to insulating members such as aluminum nitride substrates and the like mounted with silicon, they may be deformed or cracked due to the heat change in solder bonding or repeated use and due to the difference in thermal expansion between the metallic members and the semiconductor or insulating members. Therefore, materials with good heat conductivity, of which the thermal expansion approaches to that of semiconductors and insulating ceramic materials, are desired.
Aluminum nitride to be mounted with semiconductor chips is generally lined with a Cu sheet on its back surface.
As heat-radiating substrates meeting the requirements noted above, composite materials of tungsten (w)-copper (Cu) (hereinafter referred to as Wxe2x80x94Cu composites) have been proposed.
To produce such Wxe2x80x94Cu composites, employed is a method which is as follows: W powder is molded under compression to give a green compact. The green compact is then sintered in a reducing atmosphere into a porous body of W having a predetermined degree of porosity. Next, copper is infiltrated into the porous body in a reducing atmosphere at a temperature not lower than the melting point of copper to obtain a Wxe2x80x94Cu composite.
In order to evade the problem of thermal strain noted above, heat-radiating substrates for IC (integrated circuit) packages, for which are used ceramic materials, must be so designed that their thermal expansion approaches to that of alumina, beryllia and the like. For these, used are Wxe2x80x94Cu composites with from 10 to 15% by mass of copper infiltrated thereinto.
Those Wxe2x80x94Cu composites well used for heat-radiating substrates are produced by infiltrating Cu into porous bodies of W. In general, they essentially have a Cu content of from 10 to 20% by mass, and have good characteristics. For example, they have a thermal expansion coefficient of from 7 to 8xc3x9710xe2x88x926/K, and a thermal conductivity of from 180 to 200 W/mxc2x7K. However, in the recent tendency toward lightweight, thin and small parts in the art, the disadvantages of high density and heavy weight of Wxe2x80x94Cu composites are being serious problems. In addition, since Wxe2x80x94Cu composites are worked by cutting into products, their another drawback is that they could not be thinned well.
Specific methods for producing Moxe2x80x94Cu green compacts are mentioned below.
One method is known for producing green compacts of ordinary ceramics, metal powders and their composites through isostatic molding, which comprises putting a powder to be molded, for example a powder of Mo or the like, into a rubber bag mold or the like, sealing the mold, then putting it into a hydraulic pressure tank filled with water, and applying an external hydraulic press to the rubber mold so as to press the powder into a green compact.
Also known is another method for producing such green compact through ordinary powder pressing, for which is used a pressing device. The pressing device comprises a mortar to give the inner wall surface for the cavity, and upper and lower rods to give the upper and lower surfaces for the cavity. Concretely, a powder of Mo or the like is filled into the cavity to be formed by the mortar and the rods, sealed with the upper rod, and then compressed by the upper and lower rods into a green compact.
Using conventional Cuxe2x80x94Mo composites for producing heat-radiating substrates for semiconductor packages for microwaves is problematic in various aspects of, for example, the characteristics of the composites, the workability thereof, and even the thickness of the products to be produced from them.
For producing heat-radiating substrates such as those noted above, intermediate products of green compacts for them are prepared. In conventional isostatic molding methods, green compacts having been pressed with a uniform pressure could be obtained. In those, however, since flexible rubber molds or the like are used, well-shaped plates or green compacts having a specific shape are difficult to obtain.
On the other hand, in powder pressing methods for producing large-sized green compacts by applying pressure to powder from the upper and lower sides, large pressure is applied to powder. Therefore, for the methods, the mold, especially the mortar to be used must be so designed that its mechanical strength is satisfactorily high. For these reasons, the methods are defective in that the costs for the mold are high.
One object of the invention is to provide a semiconductor package for microwaves or power, for which is used a thin, heat-radiating substrate having good characteristics and workability.
Another object is to provide an isostatic molding method for producing inexpensive and well-shaped green compacts.
Still another object is to provide a method for producing heat-radiating substrates for microwave or power semiconductor packages, in which is used the isostatic molding method noted above.
According to one aspect of the invention, obtained is a semiconductor package to be mounted with semiconductor chips, which is characterized in that it has a heat-radiating substrate having a thickness of smaller than 0.4 mm of a Cuxe2x80x94Mo composite as prepared by impregnating from 30 to 40% by mass of copper (Cu) melt into a green compact of molybdenum.
Preferably, in the invention, the heat-radiating substrate is a high-reliability heat-radiating substrate characterized by having a thermal expansion coefficient of from 7.7 to 9.0xc3x9710xe2x88x926/K, a thermal conductivity of from 200 to 220 W/mxc2x7K, a Young""s modulus of from 220 to 230 GPa, and a density of not larger than 9.8 g/cm3. Also preferably, the semiconductor chips are microwave semiconductor chips.
According to another aspect of the invention, obtained is a power semiconductor package with semiconductor chips being mounted on a composite substrate of aluminum nitride as sandwiched between heat-radiating substrates of high heat conductivity metal plates, which is characterized in that at least one of said heat-radiating substrates is of substantially a Cuxe2x80x94Mo composite as prepared by impregnating copper melt into a green compact of Mo powder having been previously mixed with at most 5% by mass of Cu, in such a manner that the total Cu content of the thus-impregnated green compact may fall between 40 and 60% by mass.
According to still another aspect of the invention, there is provided an isostatic molding method for producing green compacts, which comprises disposing at least two plates adjacent to the inner surface of a side wall as divided into at least two portions, putting a powder into the space between the plates with covering the powder compact with a flexible cover to prepare a composite, then putting the resulting composite into a pressure tank, applying an external isostatic pressure thereto against the flexible cover, and then sliding the plates via the cover along the side wall thereby compressing the composite between the thus-slid plates into a green compact.
In the invention, preferably, the composite is, in place of being put into the pressure tank, put into a pressure chamber in a dry-type isostatic molding device, and compressed therein into a green compact.
Also preferably, in the invention, at least two plates are so disposed that a pair of the plates face each other along the inner surface of the side wall as divided into at least two portions, and a powder is filled into the space between the plates to give a powder compact therebetween. In that condition, at least one partitioning plate is disposed between the pair of opposed plates, and a powder is filled into every space as formed by the opposed plates and the partitioning plate to give a plurality of green compacts all at a time.
Further preferably, in the invention, the powder is of Mo to give a green compact of Mo.
According to still another aspect of the invention, there is provided a method for producing heat-radiating substrates, which comprises disposing at least two plates adjacent to the inner surface of a side wall as divided into at least two portions, putting Mo powder into the space between the plates with covering the powder compact with a flexible cover to prepare a composite, then putting the resulting composite into a pressure tank, applying an external isostatic pressure thereto against the flexible cover, then sliding the plates via the cover along the side wall thereby compressing the composite between the thus-slid plates into an Mo green compact, mounting Cu on the Mo green compact, and then heating the Mo green compact with Cu mounted thereon so as to impregnate Cu into the Mo green compact to give a heat-radiating, Cuxe2x80x94Mo composite substrate.
In the invention, preferably, the Cuxe2x80x94Mo composite substrate is rolled into a heat-radiating substrate having a thickness of smaller than 0.4 mm.